Dithiaarsanes Induce Oxidative Stress-Mediated Apoptosis in HL-60 Cells by Selectively Targeting Thioredoxin Reductase

A fast and specific fluorescent probe for thioredoxin reductase that works via disulphide bond cleavage

Small molecule probes are indispensable tools to explore diverse cellular events. However, finding a specific probe of a target remains a high challenge. Here we report the discovery of Fast-TRFS, a specific and superfast fluorogenic probe of mammalian thioredoxin reductase, a ubiquitous enzyme involved in regulation of diverse cellular redox signaling pathways. By systematically examining the processes of fluorophore release and reduction of cyclic disulfides/diselenides by the enzyme, structural factors that determine the response rate and specificity of the probe are disclosed. Mechanistic studies reveal that the fluorescence signal is switched on by a simple reduction of the disulfide bond within the probe, which is in stark contrast to the sensing mechanism of published probes. The favorable properties of Fast-TRFS enable development of a high-throughput screening assay to discover inhibitors of thioredoxin reductase by using crude tissue extracts as a source of the enzyme.

Thioredoxin reductase (TrxR) plays a crucial part in regulating cellular redox homeostasis. Here, the authors developed a fluorescent probe composed of a five-membered disulphide, a coumarin fluorophore and a urea linker that detects TrxR activity with fast response and high selectivity.

Xanthohumol Analogues as Potent Nrf2 Activators against Oxidative Stress Mediated Damages of PC12 Cells

The nuclear factor erythroid 2-related factor 2 (Nrf2), a master transcription factor controlling a series of cytoprotective genes, is closely associated with scavenging the reactive oxygen species and maintaining the intracellular redox balance. Accumulating evidence has indicated that activation of Nrf2 is efficient to block or retard oxidative stress mediated neurodegenerative disorders. Small molecules that contribute directly or indirectly to the Nrf2 activation thus are promising therapeutic agents. Herein, we screened xanthohumol and its analogues, and two analogues (11 and 12) were disclosed to possess low cytotoxicity and rescue PC12 cells from the hydrogen peroxide or 6-hydroxydopamine induced injuries. Molecular mechanism studies demonstrated that compounds 11 and 12 are potent Nrf2 activators by promoting the nuclear accumulation of Nrf2 and enhancing the cellular antioxidant defense system. More importantly, genetically silencing the Nrf2 expression shuts down the observed cytoprotection conferred by both compounds, supporting the critical involvement of Nrf2 for the cellular actions of compounds 11 and 12.

Curcuminoid WZ35 synergize with cisplatin by inducing ROS production and inhibiting TrxR1 activity in gastric cancer cells

Background

Cisplatin is one of the most widely used chemotherapeutic agents, but its efficacy is limited by its side effects. Hence, it is of great significance to develop novel agents to synergize with cisplatin and decrease side effects. In our previous study, we demonstrated that WZ35, a novel curcumin analogue, exhibited potent anti-cancer effects in vitro and in vivo. Here, we investigated whether WZ35 synergize to potentiate cisplatin activity in gastric cancer cells.

Methods

Cell apoptosis and cellular ROS levels were analyzed by flow cytometry. TrxR1 activity in gastric cells or tumor tissues was determined by the endpoint insulin reduction assay. Western blot was used to analyze the levels of indicated molecules. Nude mice xenograft model was used to test the effects of WZ35 and cisplatin combination on gastric cancer cell growth in vivo.

Results

We found that WZ35 significantly enhanced cisplatin-induced cell growth inhibition and apoptosis in gastric cancer cells. Further mechanism study showed that WZ35 synergized the anti-tumor effects of cisplatin by inhibiting TrxR1 activity. By inhibiting TrxR1 activity, WZ35 combined with cisplatin markedly induced the production of ROS, activated p38 and JNK signaling pathways, and eventually induced apoptosis of gastric cancer cells. In vivo, WZ35 combined with cisplatin significantly suppressed tumor growth in a gastric cancer xenograft model, and effectively reduced the activity of TrxR1 in tumor tissues. Remarkably, WZ35 attenuated the body weight loss evoked by cisplatin treatment.

Conclusion

This study elucidated the underlying mechanisms of synergistic effect of WZ35 and cisplatin, and suggest that such a combinational treatment might potentially become a more effective regimen in gastric cancer therapy.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1215-y) contains supplementary material, which is available to authorized users.

Promotion of HeLa cells apoptosis by cynaropicrin involving inhibition of thioredoxin reductase and induction of oxidative stress

Cancer is considered as one of the highly mortal diseases globally. This is largely due to the lack of efficacious medicines for tumors, and thus development of potent anticancer agents is urgently needed. The thioredoxin (Trx) system is crucial to the survival ability of cells and its expression is up-regulated in many human tumors. Recently, increasing evidence has been established that mammalian thioredoxin reductase (TrxR), a selenocysteine-containing protein and the core component of the thioredoxin system, is a promising therapeutic target. The sesquiterpene lactone compound cynaropicrin (CYN), a major component of Cynara scolymus L., has shown multiple pharmacological functions, especially the anticancer effect, in many experimental models. Most of these functions are concomitant with the production of reactive oxygen species (ROS). Nevertheless, the target of this promising natural anticancer product in redox control has rarely been explored. In this study, we showed that CYN induces apoptosis of Hela cells. Mechanistic studies demonstrated that CYN impinges on the thioredoxin system via inhibition of TrxR, which leads to Trx oxidation and ROS accumulation in HeLa cells. Particularly, the cytotoxicity of CYN is enhanced through the genetic knockdown of TrxR, supporting the pharmacological effect of CYN is relevant to its inhibition of TrxR. Together, our studies reveal an unprecedented mechanism accounting for the anticancer effect of CYN and identify a promising therapeutic agent worthy of further development for cancer therapy.

Neuroprotection of mangiferin against oxidative damage via arousing Nrf2 signaling pathway in PC12 cells

Accumulating evidence demonstrates that oxidative stress is involved in the pathogenesis and progression of neurodegeneration. As NF-E2-related factor 2 (Nrf2) plays a crucial role in maintaining cellular redox homeostasis, small molecules with the ability in activation of Nrf2 pathway are promising neuroprotective agents. Mangiferin (Mg) is a xanthone glucoside extracted from mangoes and papayas, and has been reported to possess multiple pharmacological activities. In this study, we investigated neuroprotective effects of Mg in the neuron-like rat pheochromocytoma cell line (PC12 cells). Mg scavenges different kinds of free radicals in vitro and attenuates hydrogen peroxide- or 6-hydroxydopamine-induced cell death. After treatment with Mg, a range of antioxidant genes governed by Nrf2 were upregulated, and the expressions and activities of these gene products were also elevated. Moreover, knockdown of Nrf2 antagonized the protective effect of Mg, indicating that Nrf2 is an essential factor in this cytoprotective process. In summary, our study demonstrates that Mg is a potent antioxidant that can provide neuroprotection against oxidative stress-mediated damage of PC12 cells. © 2019 BioFactors, 45(3):381-392, 2019.

Targeting the Thioredoxin System as a Strategy for Cancer Therapy

Thioredoxin reductase (TrxR) participates in the regulation of redox reactions in organisms. It works mainly via its substrate molecule, thioredoxin, to maintain the redox balance and regulate signal transduction, which controls cell proliferation, differentiation, death, and other important physiological processes. In recent years, increasing evidence has shown that the overactivation of TrxR is related to the development of tumors. The exploration of TrxR-targeted antitumor drugs has attracted wide attention and is expected to provide new therapies for cancer treatment. In this perspective, we highlight the specific relationship between TrxR and apoptotic signaling pathways. The cytoplasm and mitochondria both contain TrxR, resulting in the activation of apoptosis. TrxR activity influences reactive oxygen species (ROS) and further regulates the inflammatory signaling pathway. In addition, we discuss representative TrxR inhibitors with anticancer activity and analyze the challenges in developing TrxR inhibitors as anticancer drugs.

Genistein induces apoptosis in vitro and has antitumor activity against human leukemia HL-60 cancer cell xenograft growth in vivo

Genistein, a major isoflavone compound in soybeans, has been shown to have biological activities including anti-cancer activates. In the present, we investigated the anti-leukemia activity of genistein on HL-60 cells in vitro. The percentage of viable cell, cell cycle distribution, apoptotic cell death, reactive oxygen species (ROS), and Ca²⁺ production and the level of ΔΨ_m were measured by flow cytometric assay. Cell apoptosis and endoplasmic reticulum (ER) stress associated protein expressions were examined by Western blotting assay. Calpain 1, GRP78, and GADD153 expression were measured by confocal laser microscopy. Results indicated that genistein-induced cell morphological changes, decreased the total viable cells, induced G₂ /M phase arrest and DNA damage and fragmentation (cell apoptosis) in HL-60 cells. Genistein promoted ROS and Ca²⁺ productions and decreased the level of ΔΨ_m in HL-60 cells. Western blotting assay demonstrated that genistein increased ER stress-associated protein expression such as IRE-1α, Calpain 1, GRP78, GADD153, caspase-7, caspase-4, and ATF-6α at 20-50 μM treatment and increased apoptosis associated protein expression such as pro-apoptotic protein Bax, PARP-cleavage, caspase-9, and -3, but decreased anti-apoptotic protein such as Bcl-2 and Bid in HL-60 cells. Calpain 1, GRP78, and GADD153 were increased in HL-60 cells after exposure to 40 μM of genistein. In animal xenografted model, mice were intraperitoneally injected with genistein (0, 0.2, and 0.4 mg/kg) for 28 days and the body weight and tumor volume were recorded. Results showed that genistein did not affect the body weights but significantly reduced the tumor weight in 0.4 mg/kg genistein-treated group. Genistein also increased the expressions of ATF-6α, GRP78, Bax, Bad, and Bak in tumor. In conclusion, genistein decreased cell number through G₂ /M phase arrest and the induction of cell apoptosis through ER stress- and mitochondria-dependent pathways in HL-60 cells and suppressed tumor properties in vivo.

Virtual screening-guided discovery of thioredoxin reductase inhibitors

The selenoprotein thioredoxin reductase (TXNRD) is a promising therapeutic target for cancer. To discover novel TXNRD inhibitors, a library of α, β-unsaturated carbonyl compounds were applied in structure-based virtual screening for the selection of hit compounds. Fifteen top-ranked compounds were further validated experimentally, exhibiting potent inhibition of TXNRD and remarkable cytotoxicity to cancer cells. The further binding mode analysis indicated that multiple noncovalent interactions between the inhibitors and the active pocket of TXNRD facilitated the formation of covalent bonds between the Sec498 on TXNRD and the α, β-unsaturated carbonyl groups on inhibitors. Results from both simulations and experiments demonstrated that Sec498 is the prime interaction site for the inhibition of TXNRD. Taking compound 7 as an example, the inhibition of TXNRD by compounds promoted oxidative stress-mediated apoptosis of cancer cells. Given these findings, novel TXNRD inhibitors may be discovered and introduced to the growing fields of small molecule drugs against TXNRD.

Peperomin E and its orally bioavailable analog induce oxidative stress-mediated apoptosis of acute myeloid leukemia progenitor cells by targeting thioredoxin reductase

The early immature CD34⁺ acute myeloid leukemia (AML) cell subpopulation-acute myeloid leukemia progenitor cells (APCs), is often resistant to conventional chemotherapy, making them largely responsible for the relapse of AML. However, to date, the eradication of APCs remains a major challenge. We previously reported a naturally occurring secolignan- Peperomin E (PepE) and its analog 6-methyl (hydroxyethyl) amino-2, 6-dihydropeperomin E (DMAPE) that selectively target and induce oxidative stress-mediated apoptosis in KG-1a CD34⁺ cells (an APCs-like cell line) in vitro. We therefore further evaluated the efficacy and the mechanism of action of these compounds in this study. We found that PepE and DMAPE have similar potential to eliminate primary APCs, with no substantial toxicities to the normal cells in vitro and in vivo. Mechanistically, these agents selectively inhibit TrxR1, an antioxidant enzyme aberrantly expressed in APCs, by covalently binding to its selenocysteine residue at the C-terminal redox center. TrxR1 inhibition mediated by PepE (DMAPE) leads to the formation of cellular selenium compromised thioredoxin reductase-derived apoptotic protein (SecTRAP), oxidation of Trx, induction of oxidative stress and finally activation of apoptosis of APCs. Our results demonstrate a potential anti-APCs molecular target – TrxR1 and provide valuable insights into the mechanism underlying PepE (DMAPE)-induced cytotoxicity of APCs, and support the further preclinical investigations on PepE (DMAPE)-related therapies for the treatment of relapsed AML.

Structure, Mechanism, and Inhibition of Aspergillus fumigatus Thioredoxin Reductase

Aspergillus fumigatus infections are associated with high mortality rates and high treatment costs. Limited available antifungals and increasing antifungal resistance highlight an urgent need for new antifungals. Thioredoxin reductase (TrxR) is essential for maintaining redox homeostasis and presents as a promising target for novel antifungals. We show that ebselen [2-phenyl-1,2-benzoselenazol-3(2H)-one] is an inhibitor of A. fumigatus TrxR (K_i = 0.22 μM) and inhibits growth of Aspergillus spp., with in vitro MIC values of 16 to 64 µg/ml. Mass spectrometry analysis demonstrates that ebselen interacts covalently with a catalytic cysteine of TrxR, Cys148. We also present the X-ray crystal structure of A. fumigatus TrxR and use in silico modeling of the enzyme-inhibitor complex to outline key molecular interactions. This provides a scaffold for future design of potent and selective antifungal drugs that target TrxR, improving the potency of ebselen toward inhbition of A. fumigatus growth.

A mitochondria-targeted organic arsenical accelerates mitochondrial metabolic disorder and function injury

Considering the vital role of mitochondria in the anti-cancer mechanism of organic arsenical, the mitochondria-targeted precursor PDT-PAO-TPP was designed and synthesized. PDT-PAO-TPP, as a delocalization lipophilic cation (DLCs) which mainly accumulated in mitochondria, contributed to improve anti-cancer efficacy and selectivity towards NB4 cells. In detail, PDT-PAO-TPP inhibited the activity of PDHC resulting in the suppression of ATP synthesis and thermogenesis disorder. Additionally, the inhibition of respiratory chain complex I and IV by short-time incubation of PDT-PAO-TPP also accelerated the respiration dysfunction and continuous generation of ROS. These results led to the release of cytochrome c and activation of caspase family-dependent apoptosis. Different from the mechanism of PDT-PAO in HL-60 cells, it mainly induced the mitochondrial metabolic disturbance resulting in the intrinsic apoptosis via inhibiting the activity of PDHC in NB4 cells, which also implied that the efficacy exertion of organic arsenical was a complex process involved in many aspects of cellular function. This study systematically clarifies the anti-cancer mechanism of mitochondria-targeted organic arsenical PDT-PAO-TPP and confirms the new target PDHC of organic arsenicals, which further supports the organic arsenical as a promising anticancer drug.

Activation of Nrf2 by costunolide provides neuroprotective effect in PC12 cells

Costunolide (COS), a natural sesquiterpene lactone originally isolated from Inula helenium (Compositae), shows potent neuroprotective effects against oxidative stress-mediated injuries of PC12 cells via activating transcription factor Nrf2.

Honokiol Alleviates Oxidative Stress-Induced Neurotoxicity via Activation of Nrf2

Honokiol (Hon), a polyphenol and main active ingredient from the bark of Magnolia officinalis, has been documented as having multiple pharmacological functions, including neuroprotection. However, the mechanisms underlying its neuroprotective effects are not well-defined. In this study, we reported that Hon attenuates the H₂O₂- or 6-hydroxydopamine (6-OHDA)-induced apoptosis of PC12 cells by increasing the glutathione level and upregulating a multitude of cytoprotective proteins, including heme oxygenase 1, NAD(P)H:quinone oxidoreductase 1, thioredoxin 1, and thioredoxin reductase 1. Further studies reveal that Hon promotes transcription factor Nrf2 nuclear translocation and activation. Moreover, the cytoprotection of Hon was antagonized by silence of Nrf2 expression, highlighting the fact that Nrf2 is critically engaged in the cellular functions of Hon. Taken together, our study identified that Hon is an effective agonist of Nrf2 in the neuronal system and displays potent neuroprotection against oxidative stress-mediated PC12 cell damage. These findings indicate that Hon is promising for further development as a therapeutic drug against oxidative stress-related neurodegenerative disorders.

Endoplasmic reticulum-targeted two-photon turn-on fluorescent probe for nitroreductase in tumor cells and tissues

Hypoxia conditions could increase the activity of intracellular nitroreductase (NTR) and lead to many malignant diseases. Therefore, monitoring the activity of NTR is of great significance to study the related diseases. Organelles play crucial roles in the metabolism of living cells. In these organelles, the endoplasmic reticulum (ER) possesses single membrane structure, and it is the largest organelle in the cell. ER performs the synthesis, processing and modification of proteins and lipid, stabilizing the intracellular Ca²⁺ concentration and other physiological functions in living cells. Therefore, it is of great significance to develop ER-target probes in living system. Toward this goal, a new endoplasmic reticulum-targeted two-photon fluorescence turn-on NTR probe Na-NTR-ER is designed and synthesized. Probe Na-NTR-ER has been proved to display high sensitivity (36 ng/mL) and selectivity to NTR. Particularly, probe Na-NTR-ER has been successfully applied for the monitoring of NTR in ER with a high the Pearson's colocalization coefficient as 0.90 in HeLa cells and cancerous mouse tissues up to the depth of 100 μm with significant fluorescence signals.

Significance of the mitochondrial thioredoxin reductase in cancer cells: An update on role, targets and inhibitors

Thioredoxin reductase 2 (TrxR2) is a key component of the mitochondrial thioredoxin system able to transfer electrons to peroxiredoxin 3 (Prx3) in a reaction mediated by thioredoxin 2 (Trx2). In this way, both the level of hydrogen peroxide and thiol redox state are modulated. TrxR2 is often overexpressed in cancer cells conferring apoptosis resistance. Due to their exposed flexible arm containing selenocysteine, both cytosolic and mitochondrial TrxRs are inhibited by a large number of molecules. The various classes of inhibitors are listed and the molecules acting specifically on TrxR2 are extensively described. Particular emphasis is given to gold(I/III) complexes with phosphine, carbene or other ligands and to tamoxifen-like metallocifens. Also chemically unrelated organic molecules, including natural compounds and their derivatives, are taken into account. An important feature of many TrxR2 inhibitors is provided by their nature of delocalized lipophilic cations that allows their accumulation in mitochondria exploiting the organelle membrane potential. The consequences of TrxR2 inhibition are presented focusing especially on the impact on mitochondrial pathophysiology. Inhibition of TrxR2, by hindering the activity of Trx2 and Prx3, increases the mitochondrial concentration of reactive oxygen species and shifts the thiol redox state toward a more oxidized condition. This is reflected by alterations of specific targets involved in the release of pro-apoptotic factors such as cyclophilin D which acts as a regulator of the mitochondrial permeability transition pore. Therefore, the selective inhibition of TrxR2 could be utilized to induce cancer cell apoptosis.

A new synthetic derivative of cryptotanshinone KYZ3 as STAT3 inhibitor for triple-negative breast cancer therapy

Silencing STAT3 is confirmed as a promising therapeutic strategy for triple-negative breast cancer (TNBC) therapy to address the issue of its poor prognosis. In this study, the natural product cryptotanshinone was firstly remodeled and modified as a more effective STAT3 inhibitor by structure-based strategy. The synthetic derivative KYZ3 had 22-24-fold increase in antitumor activity than cryptotanshinone on two TNBC cell lines but had little effect on normal breast epithelial MCF-10A cells. Further investigation showed that KYZ3 inhibited persistent STAT3 phosphorylation. It also prevented the STAT3 protein nuclear translocation to regulate the expressions of the target oncogenes including Bax and Bcl-2. Furthermore, KYZ3 inhibited TNBC cell metastasis by decreasing the levels of MMP-9 which were directly regulated by activated STAT3. A STAT3 plasmid transfecting assay suggested that KYZ3 induced tumor cell apoptosis mainly by targeting STAT3. Finally, KYZ3 suppressed the growth of tumors resulting from subcutaneous implantation of MDA-MB-231 cells in vivo. Taken together, KYZ3 may be a promising cancer therapeutic agent for TNBC.

Isobavachalcone Induces ROS-Mediated Apoptosis via Targeting Thioredoxin Reductase 1 in Human Prostate Cancer PC-3 Cells

Prostate carcinoma causes a great number of deaths every year; therefore, there is an urgent need to find new drug candidates to treat advanced prostate cancer. Isobavachalcone (IBC) is the chalcone composition of Psoralea corylifolia Linn used in traditional Chinese medicine. Although IBC demonstrates potent anticancer efficacy in numerous types of human cancer cells, the cellular targets of IBC have not been fully defined. In our study, we found that IBC may induce reactive oxygen species- (ROS-) mediated apoptosis via interaction with a selenocysteine (Sec) containing the antioxidant enzyme thioredoxin reductase 1 (TrxR1), and induce lethal endoplasmic reticulum (ER) stress by inhibiting TrxR1 activity and increasing ROS levels in human prostate cancer PC-3 cells. Furthermore, we also observed that knocking down TrxR1 would sensitized cancer cells to IBC treatment. Our study provides evidence for the anticancer mechanism of IBC with TrxR1 as a potential target.

Redox-Dependent Copper Carrier Promotes Cellular Copper Uptake and Oxidative Stress-Mediated Apoptosis of Cancer Cells

Transportation of exogenous copper ions into cancer cells by copper carriers has gained increasing interest for cancer chemotherapy. We disclosed herein a redox-dependent copper carrier, 2,2'-dithiodipyridine (DPy), which binds copper ions and carries the cargo into cells. The cellular reducing environment cleaved the disulfide bond in DPy to facilitate unloading copper ions. The elevated copper level then elicits oxidative stress and subsequently promotes the reformation of DPy. Further mechanistic studies revealed that the DPy/copper combination predominantly targets the cellular redox-regulating systems, including the thioredoxin system and the glutathione system, to induce the oxidative stress-mediated death of tumor cells. The discovery of DPy as a cleavable and recyclable copper shuttle provides a proof of concept for designing novel biomaterials for copper transportation as potential anticancer agents.

Xanthatin Promotes Apoptosis via Inhibiting Thioredoxin Reductase and Eliciting Oxidative Stress

Xanthatin (XT), a naturally occurring sesquiterpene lactone presented in cocklebur ( Xanthium strumarium L.), is under development as a potential anticancer agent. Despite the promising anticancer effect of XT, the molecular mechanism underlying its cellular action has not been well elucidated. The mammalian thioredoxin reductase (TrxR) enzymes, the essential seleno-flavoproteins containing a penultimate selenocysteine (Sec) residue at the C-terminus, represent a promising target for cancer chemotherapeutic agents. In this study, XT inhibits both the purified TrxR and the enzyme in cells. The possible binding mode of XT with the TrxR protein is predicted by the covalent docking method. Mechanism studies reveal that XT targets the Sec residue of TrxR and inhibits the enzyme activity irreversibly. Simultaneously, the inhibition of TrxR by XT promotes the oxidative stress-mediated apoptosis of HeLa cells. Importantly, the knockdown of the enzyme sensitizes the cells to XT treatment. Targeting TrxR thus discloses a novel molecular mechanism in accounting for the cellular action of XT and provides insights into the development of XT as an anticancer agent.

Interaction of Arsenous Acid with the Dithiol-Type Chelator British Anti-Lewisite (BAL): Structure and Stability of Species Formed in an Unexpectedly Complex System

British anti-Lewisite (2,3-dimerkaptopropan-1-ol, dimercaprol, BAL) is one of the best-known chelator-type therapeutic agents against toxic metal ions and metalloids, especially arsenicals. Surprisingly, the mechanisms of action at the molecular level, as well as the coordination features of this traditional drug toward various arsenicals, are still poorly revealed. The present study on the interaction of arsenous acid (H₃AsO₃) with BAL, involving UV and NMR titrations, electrospray ionization mass spectrometry, and 2D NMR experiments combined with MP2 calculations, demonstrates that the reaction of H₃AsO₃ with BAL at pH = 7.0 results in a more complex speciation than was assumed before. The three reactive hydroxyl groups of H₃AsO₃ allow for interaction with three thiol moieties via condensation reaction, leading to the observed AsBAL₂ and As₂BAL₃ complexes besides the AsBAL species. This indicates the strong propensity of inorganic As(III) to saturate its coordination sphere with thiolate groups. The alcoholic hydroxyl group of the ligand may also directly bind to As(III) in AsBAL. Compared to dithiothreitol or dithioeritritol, the preference of BAL to form complexes with such a tridentate binding mode is much lower owing to the more strained bridged bicyclic structure with an α_AsSC < 90° bond angle and an unfavorable condensed boat-type six-membered ring. On the basis of the NMR data, the predominating, bidentately bound AsBAL species, including a five-membered chelate ring, exists in rapidly interconverting envelope forms of E and Z stereoisomers. The conditional stability constants calculated for the three macrospecies from a series of UV data [log β^pH=7.0 = 6.95 (AsBAL), 11.56 (AsBAL₂), and 22.73 (As₂BAL₃)] reflect that BAL is still the most efficient, known, dithiol-type chelator of H₃AsO₃.

Selective Activation of a Prodrug by Thioredoxin Reductase Providing a Strategy to Target Cancer Cells

Elevated reactive oxygen species and antioxidant defense systems have been recognized as one of the hallmarks of cancer cells. As a major regulator of the cellular redox homeostasis, the selenoprotein thioredoxin reductase (TrxR) is increasingly considered as a promising target for anticancer drug development. The current approach to inhibit TrxR predominantly relies on the modification of the selenocysteine residue in the C-terminal active site of the enzyme, in which it is hard to avoid the off-target effects. By conjugating the anticancer drug gemcitabine with a 1,2-dithiolane scaffold, an unprecedented prodrug strategy is disclosed that achieves a specific release of gemcitabine by TrxR in cells. As overexpression of TrxR is frequently found in different types of tumors, the TrxR-dependent prodrugs are promising for further development as cancer chemotherapeutic agents.

Carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline as a cytotoxic agent against colorectal cancer

Arsenic trioxide (As₂O₃) has been approved for the treatment of acute promyelocytic leukemia (APL); however, its use in the treatment of solid tumors is limited due to its pharmacokinetic properties. Organic arsenic compounds provide better options for pharmaceutical optimization. p-Aminophenyl arsenoxide (p-APAO), an organic arsenic compound, was found to interact with the promyelocytic leukemia–retinoic acid receptor alpha (PML–RARα) fusion protein in a similar manner to arsenic trioxide. Analogs of p-APAO such as 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) were recently found to show improved cytotoxicity toward several solid tumor cell lines with lower toxicity to normal cells. Here, we synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline (p-APDTAs) and showed that it exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

We synthesized a carbohydrate-conjugated 4-(1,3,2-dithiarsolan-2-yl)aniline. It exhibited reduced cytotoxicity to normal cells, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

Mitochondrial toxicity of organic arsenicals: membrane permeability transition pore opening and respiratory dysfunction

In order to clarify the mitochondrial toxicity mechanism of the organic arsenical MOPIMP (2-methoxy-4-(((4-(oxoarsanyl) phenyl) imino) methyl) phenol), research was carried out at the sub-cell level based on the previous finding that the compound MOPIMP can damage the mitochondria by triggering a burst of ROS. After investigating its influence on isolated mitochondria in vitro, it was demonstrated that a high dose of MOPIMP with short-term exposure can induce mitochondrial swelling, decrease the membrane potential, enhance the permeability of H⁺ and K⁺, and induce membrane lipid peroxidation, indicating that it can result in an MPT process in a ROS-mediated and Ca²⁺-independent manner. Additionally, MPT was also aggravated as a result of impairment of the membrane integrity and membrane fluidity. In addition, short-term incubation between mitochondria and compound MOPIMP promoted the inhibition of respiratory chain complexes I, II, III and IV, as well as damage to the respiration process, which supported the previous finding about the burst of ROS. On the other hand, after long-term exposure by the organic arsenical MOPIMP, mitochondrial metabolic dysfunction was triggered, which was in accordance with perturbation of the respiratory chain complexes as well as the respiration process. This work systematically sheds light on the mitochondrial toxicity mechanism of the organic arsenical MOPIMP, including induction of the MPT process and inhibition of respiratory metabolism, which provides a potential target for organic arsenicals as anti-tumor drugs.

10H-3,6-Diazaphenothiazine induces G2/M phase cell cycle arrest and caspase-dependent apoptosis and inhibits cell invasion of A2780 ovarian carcinoma cells through the regulation of NF-κB and (BIRC6-XIAP) complexes

The asymptomatic properties and high treatment resistance of ovarian cancer result in poor treatment outcomes and high mortality rates. Although the fundamental chemotherapy provides promising anticancer activities, it is associated with severe side effects. The derivative of phenothiazine, namely, 10H-3,6-diazaphenothiazine (PTZ), was synthesized and reported with ideal anticancer effects in a previous paper. In this study, detailed anticancer properties of PTZ was examined on A2780 ovarian cancer cells by investigating the cytotoxicity profiles, mechanism of apoptosis, and cell invasion. Research outcomes revealed PTZ-induced dose-dependent inhibition on A2780 cancer cells (IC₅₀ =0.62 µM), with significant less cytotoxicity toward HEK293 normal kidney cells and H9C2 normal heart cells. Generation of reactive oxygen species (ROS) and polarization of mitochondrial membrane potential (ΔΨm) suggests PTZ-induced cell death through oxidative damage. The RT² Profiler PCR Array on apoptosis pathway demonstrated PTZ-induced apoptosis via intrinsic (mitochondria-dependent) and extrinsic (cell death receptor-dependent) pathway. Inhibition of NF-κB and subsequent inhibition of (BIRC6-XIAP) complex activities reduced the invasion rate of A2780 cancer cells penetrating through the Matrigel™ Invasion Chamber. Lastly, the cell cycle analysis hypothesizes that the compound is cytostatic and significantly arrests cell proliferation at G₂/M phase. Hence, the exploration of the underlying anticancer mechanism of PTZ suggested its usage as promising chemotherapeutic agent.

Synthesis of naphthazarin derivatives and identification of novel thioredoxin reductase inhibitor as potential anticancer agent

Mammalian thioredoxin reductase (TrxR) enzymes play a crucial role in regulating multiple redox-based signaling pathways and have attracted increasing attention as promising anticancer drug targets. We report here the synthesis of a panel of naphthazarin derivatives and discovery of 2-methyl-5,8-dihydroxy-1,4-naphthoquinone (3, 2-methylnaphthazarin) as a potent cytotoxic agent with a submicromolar half maximal inhibitory concentration to the human promyelocytic leukemia HL-60 cells. Mechanism studies reveal that the compound selectively inhibits TrxR to induce oxidative stress-mediated apoptosis of HL-60 cells. Knockdown of TrxR sensitizes the cells to 3 insults, while overexpression of the functional enzyme confers resistance to the compound treatment, underpinning the physiological significance of targeting TrxR by 3. Clarification of the interaction of compound 3 with TrxR unveils a mechanism underlying the cellular action of the compound, and sheds light in considering development of the compound as a potential cancer chemotherapeutic agent.

Activation of Nrf2-driven antioxidant enzymes by cardamonin confers neuroprotection of PC12 cells against oxidative damage

Oxidative stress represents a disorder of the redox equilibrium between the production of free radicals and the capability of cells to eliminate them. As subversion of this redox balance is thought to initiate various diseases, living cells maintain a redox equilibrium diligently. More and more pieces of evidence show that oxidative stress has already become a common risk factor in the pathogenesis of neurodegenerative disorders. So, considerable importance has been given to the prevention of oxidative stress as a potential therapeutic strategy. It is well known that the Nrf2-ARE pathway represents one of the most important cellular endogenous defense mechanisms against oxidative stress. Activation of Nrf2 signaling induces the transcriptional regulation of multiple ARE-dependent antioxidant defense genes. Here, we showed that cardamonin (CD), a chalcone isolated from Alpinia katsumadai, attenuated cell death induced by hydrogen peroxide (H₂O₂) and 6-hydroxydopamine (6-OHDA) in PC12 cells. Pretreatment of PC12 cells with CD dose-dependently upregulated the expression of phase II antioxidant molecules governed by Nrf2. In contrast, CD failed to provide neuroprotection after silencing Nrf2 expression, indicating that this cytoprotection may be mediated by the activation of transcription factor Nrf2. Our results demonstrate that CD is a novel small molecule activator of Nrf2 in PC12 cells, and suggest that CD may be a potential candidate for the prevention of oxidative stress-mediated neurodegenerative disorders.

Targeting the Thioredoxin System for Cancer Therapy

Thioredoxin (Trx) and thioredoxin reductase (TrxR) are essential components of the Trx system which plays pivotal roles in regulating multiple cellular redox signaling pathways. In recent years TrxR/Trx have been increasingly recognized as an important modulator of tumor development, and hence targeting TrxR/Trx is a promising strategy for cancer treatment. In this review we first discuss the structural details of TrxR, the functions of the Trx system, and the rational of targeting TrxR/Trx for cancer treatment. We also highlight small-molecule TrxR/Trx inhibitors that have potential anticancer activity and review their mechanisms of action. Finally, we examine the challenges of developing TrxR/Trx inhibitors as anticancer agents and perspectives for selectively targeting TrxR/Trx.

Curcumin analog L48H37 induces apoptosis through ROS-mediated endoplasmic reticulum stress and STAT3 pathways in human lung cancer cells

Lung cancer is the leading cause of cancer-related deaths. Curcumin is a well-known natural product with anticancer ability, however, its poor bioavailability and pharmacokinetic profiles have limited its application in anticancer therapy. Previously, we reported that L48H37, a novel analog of curcumin with higher bioavailability, ameliorated LPS-induced inflammation, but the anticancer effect of L48H37 is still unknown. In the present study, we have investigated the effects of L48H37 in human lung cancer cells. Our results show that L48H37 decreases lung cancer cell growth and colony formation. These alterations were mediated through induction of G2/M cell cycle arrest and apoptosis in lung cancer cells. After L48H37 treatment, ER stress-related proteins were increased, and the expression of p-STAT3 was decreased in a dose-dependent manner. L48H37 also induced the accumulation of ROS in lung cancer cells, and pretreatment with NAC could fully reverse L48H37-induced reactive oxygen species (ROS) increase. Blocking ROS was able to reverse L48H37-induced endoplasmic reticulum (ER) stress, cell cycle arrest, and apoptosis. Finally, we show that L48H37 inhibits the growth of lung cancer xenografts without exhibiting toxicity. Treatment of mice bearing human lung cancer xenografts with L48H37 was also associated with indices of ER stress activation. In summary, our results provide evidence for a novel anti-tumor candidate for the treatment of lung cancer.

EF24 induces ROS-mediated apoptosis via targeting thioredoxin reductase 1 in gastric cancer cells

Gastric cancer (GC) is one of the leading causes of cancer mortality in the world, and finding novel agents for the treatment of advanced gastric cancer is of urgent need. Diphenyl difluoroketone (EF24), a molecule having structural similarity to curcumin, exhibits potent anti-tumor activities by arresting cell cycle and inducing apoptosis. Although EF24 demonstrates potent anticancer efficacy in numerous types of human cancer cells, the cellular targets of EF24 have not been fully defined. We report here that EF24 may interact with the thioredoxin reductase 1 (TrxR1), an important selenocysteine (Sec)-containing antioxidant enzyme, to induce reactive oxygen species (ROS)-mediated apoptosis in human gastric cancer cells. By inhibiting TrxR1 activity and increasing intracellular ROS levels, EF24 induces a lethal endoplasmic reticulum stress in human gastric cancer cells. Importantly, knockdown of TrxR1 sensitizes cells to EF24 treatment. In vivo, EF24 treatment markedly reduces the TrxR1 activity and tumor cell burden, and displays synergistic lethality with 5-FU against gastric cancer cells. Targeting TrxR1 with EF24 thus discloses a previously unrecognized mechanism underlying the biological activity of EF24, and reveals that TrxR1 is a good target for gastric cancer therapy.

Potent anticancer activity of a new bismuth (III) complex against human lung cancer cells

The aim of this work is experimental study of an interesting bismuth(III) complex derived from pentadentate 2,6-pyridinedicarboxaldehyde bis(⁴N-methylthiosemicarbazone), [BiL(NO₃)₂]NO₃ {L=2,6-pyridinedicarboxaldehyde bis(⁴N-methylthiosemicarbazone)}. A series of in vitro biological studies indicate that the newly prepared [BiL(NO₃)₂]NO₃ greatly suppressed colony formation, migration and significantly induced apoptosis of human lung cancer cells A549 and H460, but did not obviously decrease the cell viability of non-cancerous human lung fibroblast (HLF) cell line, showing much higher anticancer activities than its parent ligands, especially with half maximum inhibitory concentration (IC₅₀) <3.5μM. Moreover, in vivo study provides enough evidence that the treatment with [BiL(NO₃)₂]NO₃ effectively inhibited A549 xenograft tumor growth on tumor-bearing mice (10mgkg^-1, tumor volume reduced by 97.92% and tumor weight lightened by 94.44% compared to control) and did not indicate harmful effect on mouse weight and liver. These results suggest that the coordination of free ligand with Bi(III) might be an interesting and potent strategy in the discovery of new anticancer drug candidates.

Oxidative stress-mediated intrinsic apoptosis in human promyelocytic leukemia HL-60 cells induced by organic arsenicals

Arsenic trioxide has shown the excellent therapeutic efficiency for acute promyelocytic leukemia. Nowadays, more and more research focuses on the design of the arsenic drugs, especially organic arsenicals, and on the mechanism of the inducing cell death. Here we have synthesized some organic arsenicals with Schiff base structure, which showed a better antitumor activity for three different kinds of cancer cell lines, namely HL-60, SGC 7901 and MCF-7. Compound 2a (2-(((4-(oxoarsanyl)phenyl)imino)methyl)phenol) and 2b (2-methoxy-4-(((4-(oxoarsanyl)phenyl)imino)methyl)phenol) were chosen for further mechanism study due to their best inhibitory activities for HL-60 cells, of which the half inhibitory concentration (IC₅₀) were 0.77 μM and 0.51 μM, respectively. It was illustrated that 2a or 2b primarily induced the elevation of reactive oxygen species, decrease of glutathione level, collapse of mitochondrial membrane potential, release of cytochrome c, activation of Caspase-3 and apoptosis, whereas all of the phenomena can be eliminated by the addition of antioxidants. Therefore, we concluded that compound 2a and 2b can induce the oxidative stress-mediated intrinsic apoptosis in HL-60 cells. Both the simplicity of structure with Schiff base group and the better anticancer efficiency demonstrate that organic arsenicals are worthy of further exploration as a class of potent antitumor drugs.

EM23, A Natural Sesquiterpene Lactone from Elephantopus mollis, Induces Apoptosis in Human Myeloid Leukemia Cells through Thioredoxin- and Reactive Oxygen Species-Mediated Signaling Pathways

Elephantopus mollis (EM) is a traditional herbal medicine with multiple pharmacological activities. However, the efficacy of EM in treating human leukemia is currently unknown. In the current study, we report that EM23, a natural sesquiterpene lactone isolated from EM, inhibits the proliferation of human chronic myeloid leukemia (CML) K562 cells and acute myeloid leukemia (AML) HL-60 cells by inducing apoptosis. Translocation of membrane-associated phospholipid phosphatidylserines, changes in cell morphology, activation of caspases, and cleavage of PARP were concomitant with this inhibition. The involvement of the mitochondrial pathway in EM23-mediated apoptosis was suggested by observed disruptions in mitochondrial membrane potential. Mechanistic studies indicated that EM23 caused a marked increase in the level of reactive oxygen species (ROS). Pretreatment with N-acetyl-L-cysteine, a ROS scavenger, almost fully reversed EM23-mediated apoptosis. In EM23-treated cells, the expression levels of thioredoxin (Trx) and thioredoxinreductase (TrxR), two components of the Trx system involved in maintaining cellular redox homeostasis, were significantly down-regulated. Concomitantly, Trx regulated the activation of apoptosis signal-regulating kinase 1 (ASK1) and its downstream regulatory targets, the p38, JNK, and ERK MAPKs. EM23-mediated activation of ASK1/MAPKs was significantly inhibited in the presence of NAC. Furthermore, tumor necrosis factor alpha (TNF-α)-mediated activation of nuclear factor-κB (NF-κB) was suppressed by EM23, as suggested by the observed blockage of p65 nuclear translocation, phosphorylation, and reversion of IκBα degradation following EM23 treatment. Taken together, these results provide important insights into the anticancer activities of the EM component EM23 against human CML K562 cells and AML HL-60 cells.

Targeting Thioredoxin Reductase by Parthenolide Contributes to Inducing Apoptosis of HeLa Cells

Parthenolide (PTL), a major active sesquiterpene lactone from the herbal plant Tanacetum parthenium, has been applied in traditional Chinese medicine for centuries. Although PTL demonstrates potent anticancer efficacy in numerous types of malignant cells, the cellular targets of PTL have not been well defined. We reported here that PTL interacts with both cytosolic thioredoxin reductase (TrxR1) and mitochondrial thioredoxin reductase (TrxR2), two ubiquitous selenocysteine-containing antioxidant enzymes, to elicit reactive oxygen species-mediated apoptosis in HeLa cells. PTL selectively targets the selenocysteine residue in TrxR1 to inhibit the enzyme function, and further shifts the enzyme to an NADPH oxidase to generate superoxide anions, leading to reactive oxygen species accumulation and oxidized thioredoxin. Under the conditions of inhibition of TrxRs in cells, PTL does not cause significant alteration of cellular thiol homeostasis, supporting selective target of TrxRs by PTL. Importantly, overexpression of functional TrxR1 or Trx1 confers protection, whereas knockdown of the enzymes sensitizes cells to PTL treatment. Targeting TrxRs by PTL thus discloses an unprecedented mechanism underlying the biological activity of PTL, and provides deep insights to understand the action of PTL in treatment of cancer.

Discovery of New Monocarbonyl Ligustrazine-Curcumin Hybrids for Intervention of Drug-Sensitive and Drug-Resistant Lung Cancer

The elevation of oxidative stress preferentially in cancer cells by inhibiting thioredoxin reductase (TrxR) and/or enhancing reactive oxygen species (ROS) production has emerged as an effective strategy for selectively targeting cancer cells. In this study, we designed and synthesized 21 ligustrazine-curcumin hybrids (10a-u). Biological evaluation indicated that the most active compound 10d significantly inhibited the proliferation of drug-sensitive (A549, SPC-A-1, LTEP-G-2) and drug-resistant (A549/DDP) lung cancer cells but had little effect on nontumor lung epithelial-like cells (HBE). Furthermore, 10d suppressed the TrxR/Trx system and promoted intracellular ROS accumulation and cancer cell apoptosis. Additionally, 10d inhibited the NF-κB, AKT, and ERK signaling, P-gp-mediated efflux of rhodamine 123, P-gp ATPase activity, and P-gp expression in A549/DDP cells. Finally, 10d repressed the growth of implanted human drug-resistant lung cancer in mice. Together, 10d acts a novel TrxR inhibitor and may be a promising candidate for intervention of lung cancer.

Inhibition of thioredoxin reductase by alantolactone prompts oxidative stress-mediated apoptosis of HeLa cells

The mammalian thioredoxin reductase (TrxR) isoenzymes, TrxR1 in cytosol or nucleus, TrxR2 in mitochondria, and TrxR3 in testis, are essential seleno-flavoenzymes with a conserved penultimate selenocysteine (Sec) residue at the C-terminus, and have attracted increasing interests as potential targets for development of cancer chemotherapeutic agents. The sesquiterpene lactone alantolactone (ATL), an active component from the traditional folk medicine Inula helenium, has been documented possessing multiple pharmacological functions, especially the anticancer activity. However, the underlying mechanism has not been well defined. We reported that ATL inhibits both the recombinant TrxR and the enzyme in the cellular environment. The alpha-methylene-gamma-lactone moiety in ATL and the Sec residue in TrxR are critical for targeting TrxR by ATL. By employing our newly developed pull down assay, we demonstrated the remarkable elevation of the oxidized thioredoxin in HeLa cells after ATL treatment. In addition, ATL elicits accumulation of reactive oxygen species, and eventually induces apoptosis of HeLa cells. Importantly, overexpression of the functional TrxR attenuates the cytotoxicity of ATL, while knockdown of the enzyme sensitizes the cells to ATL treatment. Targeting TrxR thus discloses a novel molecular mechanism underlying the cellular action of ATL, and sheds light in considering the usage of ATL as a potential cancer chemotherapeutic agent.

A fast response and red emission probe for mammalian thioredoxin reductase

The first red emission probe, TRFS-red, for thioredoxin reductase was reported with improved response rate and sensitivity.

A small molecule probe reveals declined mitochondrial thioredoxin reductase activity in a Parkinson's disease model

The first off–on probe, Mito-TRFS, for imaging the mitochondrial thioredoxin reductase (TrxR2) in live cells was reported.

Development of a red fluorescent light-up probe for highly selective and sensitive detection of vicinal dithiol-containing proteins in living cells

Vicinal dithiol-containing proteins (VDPs) play a key role in cellular redox homeostasis and are responsible for many diseases. Here, we develop a red fluorescent light-up probe FAsH for the highly selective and sensitive detection of VDPs using the environment-sensitive 2-(4-dimethylaminophenyl)-4-(2-carboxyphenyl)-7-diethylamino-1-benzopyrylium (F1) as the fluorescent reporter and cyclic dithiaarsane as the targeting unit. FAsH is almost nonfluorescent in aqueous solution. However, it exhibits intense fluorescence emission upon binding to reduced bovine serum albumin (rBSA, selected as the model protein). The fluorescence intensity of FAsH is directly proportional to the concentration of rBSA over the range of 0.06-0.9 μM, with a detection limit (3δ) of 0.015 μM. Importantly, the fast kinetics of binding between FAsH and VDPs (∼2.5 min) enables the dynamic tracing of VDPs in biological systems. Preliminary experiments show that FAsH can be used for the no-wash imaging of endogenous VDPs in living cells. In addition, our study shows that F1 presents both high environment-sensitivity and good fluorescence properties, and is promising for the development of no-wash fluorescent light-up probes for target-specific proteins in living cells.

Activation of Nrf2 target enzymes conferring protection against oxidative stress in PC12 cells by ginger principal constituent 6-shogaol

Natural compounds containing phenoxyl groups and/or Michael acceptor units appear to possess antioxidant and cytoprotective properties. The ginger principal constituent 6-shogaol (6-S) represents one of such compounds. In this study, we reported that 6-S efficiently scavenges various free radicals in vitro, and displays remarkable cytoprotection against oxidative stress-induced cell damage in the neuron-like rat pheochromocytoma cell line, PC12 cells. Pretreatment of PC12 cells with 6-S significantly upregulates a series of phase II antioxidant molecules, such as glutathione, heme oxygenase 1, NAD(P)H: quinone oxidoreductase 1, thioredoxin reductase 1, and thioredoxin 1. A mechanistic study revealed that 6-S enhanced the translocation of Nrf2 from the cytosol to the nucleus and knockdown of Nrf2 abolished such protection, indicating that this cytoprotection is mediated by the activation of the transcription factor Nrf2. Another ginger constituent 6-gingerol (6-G), having a similar structure of 6-S but lacking the alpha,beta-unsaturated ketone structure (Michael acceptor moiety), failed to shelter PC12 cells from oxidative stress. Our results demonstrate that 6-S is a novel small molecule activator of Nrf2 in PC12 cells, and suggest that 6-S might be a potential candidate for the prevention of oxidative stress-mediated neurodegenerative disorders.